Facebook says the suit is bogus, and is fighting an appeal in the case.

One key issue in the case is Facebook's refusal to allow its clicks to be audited by a third party like the IAB, the Media Ratings Council or Ernst & Young.

Speaking privately, the company's clients and competitors tell us they are aware that Facebook is non-transparent when it came to its advertising business.

None of them believed Facebook was acting improperly. And none sympathized with the suit. One said, "We trust Facebook and know that they are always working to refine their filters and to identify invalid clicks."

Another added, "I don't think they're ripping people off."

However, they also said that because Facebook is so big it is able to play by its own rules in a way that might not be healthy.

"They don't let you audit," said one client. "It's a little bit suspect. A bit of a conflict of interest. … You have to trust Facebook's numbers."

Another added, "They're not playing by the rules everyone else is playing by. It's definitely an issue that there's this 800 pound gorilla out there that isn't playing by the rules."

One major issue for advertisers is that they can only observe Facebook's clicks independently if they send traffic off the site to their own web sites. As most campaigns are designed to send traffic to the advertisers' Facebook page, those clicks remain inside Facebook - and thus invisible to outside analytics.

"A lot of campaigns are not sending traffic off site so there's no way to check," one client told us.

Shuman Ghosemajumder of Shape Security
Shuman Ghosemajumder / Shape Security
Another said, "If we are driving users to a Facebook page — then we rely on Facebook metrics (impressions, clicks, conversions, engagement …) as the click goes directly to the Facebook page and not through a redirect AND we can't fire pixels on Facebook pages like we can on external sites."

Shuman Ghosemajumder, Google's former click fraud czar who is now vp/strategy at Shape Security, told us that he knows many of the team members at Facebook who are working on click validation. "They are investing heavily in this area," he says. A third-party audit of clicks, however is a "non-trivial" event at a company, he says. It requires time and resources, and an outside company must come in and perform experiments with the internal engineers. Nonetheless, "they need to take this very seriously," he says.